For relatively high pin-counts and/or relatively high performance SoCs (System-On-Chips), solder bumping on 300 mm-wafers is moving from a development phase to a high-volume production phase. At the same time, laws are being enacted to prohibit solders including lead (Pb). To meet the demand for mass-production using lead-free solder, eutectic tin-silver (Sn—Ag) solder alloys have been proposed. In addition, tin-silver-copper (Sn—Ag—Cu) ternary alloys have been proposed to potentially provide improved thermal fatigue properties.
In the reference by Hirokazu Ezawa et al. entitled “Pb-Free Bumping By Alloying Electroplated Metal Stacks” (IEEE, 2003 Electronic Components And Technology Conference, pages 664-667), a eutectic Sn—Ag solder bumping process is discussed using two step electroplating of Ag/Sn metal stacks. In the Ezawa reference, alloying behavior of Ag/Sn and Ag/Sn/Cu electroplated metal stacks is investigated using differential scanning calorimetry (DSC) analysis. In particular, a 2 μm Ag/50 μm Sn electroplated metal stack may provide the Sn—Ag solder alloy with a composition of Sn-3.5 wt. % Ag, with a melting temperature of 227° C., determined using a single peak of DSC analysis. For a 2 μm Ag/Cu/50 μm Sn electroplated stack, as the Cu thickness increases from 0 μm to 0.8 μm, the single peak temperature may decrease to 222° C. According to the Ezawa reference, metal stacks formed by multi-step electroplating can be successfully transformed to solder alloys by reflow annealing. The disclosure of the Esawa reference is hereby incorporated herein in its entirety by reference.